Turbomachine shafts are typically supported against radial and axial loads with bearings. One type of bearing is a magnetic bearing, which supports the shaft by levitating it magnetically. Magnetic bearings are typically used to avoid friction losses and lubrication requirements of more conventional bearings. Auxiliary bearings may be provided along with the magnetic bearings and may be configured to catch the shaft in the event of a magnetic bearing failure, i.e., a “drop.” This allows the shaft to coast down to a stop, thereby minimizing wear and/or damage to the turbomachine caused by the drop. The auxiliary bearings may otherwise remain idle during normal turbomachine operation.
Magnetic bearings are typically controlled by a position feedback control loop, which receives information about the position of the shaft from position sensors and responds by altering the electrical current supplied to the magnetic bearings. This changes the strength of the magnetic field supporting the shaft, which in turn then controls the position of the shaft. Typically, the magnetic bearings and their associated feedback control loops require tuning to account for vibrations in the turbomachine support structure; otherwise, the feedback control loop may treat the vibration of the turbomachine support structure as position changes in the shaft instead of in the support structure. If the feedback control loop misinterprets the movement in this manner, the feedback control loop may react by attempting to move the shaft in phase with the vibrations in the support structure. This may compound the vibration of the support structure, potentially leading to destructive conditions.
Additionally, if the magnetic bearings drop the shaft, the auxiliary bearings are required to support axial loads up to or greater than the maximum operational thrust force of the turbomachine. For example, should the magnetic thrust bearings cease to operate, the auxiliary bearings must support the load that was supported by the magnetic thrust bearing, until the turbomachine is tripped or shut down. If external loads such as surge in a compressor exceed the capacity of the magnetic thrust bearing, then the auxiliary bearing must carry that additional load as well. Accordingly, it is desirable to test thrust bearings to their rated load capacities during operation of a test rig or the actual rotating machine, and not just under static non-rotating conditions.
Applying test loads to a rotating shaft while the shaft is magnetically suspended without affecting tuning accuracy, however, can present a challenge. One reason such loading is challenging is that conventional mechanical loading devices add a combination of mass, stiffness, and damping to the shaft. This may disrupt the aforementioned tuning of the magnetic bearings and may necessitate iterative changes to the tuning after assembly. Tuning of magnetic bearings, however, is a complex procedure because magnetic bearings are sensitive actuators that combine high-frequency response and high-force capability. Poorly-designed loading devices can change the frequency characteristics of the turbomachine to such an extent that an impracticably large number of modifications to the control algorithms are required to bridge the gap between the test conditions and conditions present in the final, assembled turbomachine deployed into the field.
What is needed, therefore, is a reliable loading method and apparatus that minimizes the tuning gap between the test assembly and the actual assembly.